In order to minimize air pollution by power plants it is known to separate from the waste gas of such plants the particulate material or fly ash in a cyclone or the like and mix it with a binder. This binder is then agglomerated with or without the addition of other fillers and converted into briquets. In such a system the fly ash itself constitutes part of the aggregate of such briquets.
This has proven to be successful but does not eliminate gaseous pollutants, such as SO.sub.2, SO.sub.3, and HF, which must be removed from the waste gas. It is known in order to remove these aforementioned gaseous pollution substances to admix the flue gas with lime and then scrub it with water. The flue gas is treated with an amount of lime, in form of CaO or Ca(OH).sub.2, necessary to eliminate the gaseous pollutants. After this procedure and after washing a sludge is obtained which mainly comprises CaSO.sub.4 and CaSO.sub.3, and in case of a coal-burning plant also flue dust or fly ash.
In big plants this sludge accumulates in large quantities. Its disposal creates a problem. A purposeful utilization has not been suggested.